The cooling of electrical conductors for heavy-duty power transmission lines is required in many instances to dissipate the heat generated by the flow of electrical current therethrough. Past attempts to utilize evaporative coolants for this purpose are generally inefficient because of the fact that the coolant is directed through the conductors in the liquid and vapor phases, thereby increasing the viscosity of the coolant, and thereby the friction between the coolant and the inner wall surfaces of the conductors. This result is undesirable because it increases pressure drop and hence, the load on the pumping means which is used to force the coolant through the conductors limits the total length of the conductor which can be used with a given pump capacity, and through the fluid-wall friction produces nonnegligible heating. Additionally, the nonsegrated two-phase liquid-vapor is an inefficient cooling fluid as the vapor can support a significant temperature gradient relative to the liquid. This allows the temperature of the vapor and surrounding conductors to rise above that of the boiling point of the liquid.
Another approach is to use the same fluid as both the coolant flowing through the conductor and the dielectric between the conductor and an external shield surrounding the same, as described by the Battelle Northwest Laboratory. In this case, the coolant flows in the liquid state within the conductor, and some of the resulting vapor is directed into the space surrounding the conductor within the shield. In the vapor state, the substance is supposed to act as a dielectric to prevent voltage breakdown between the conductor and the shield. No discussion of the undesirable effects of increased viscosity, diminished heat transfer, and concomitant increased temperature was made. Therefore, it appears that the Battelle vapor venting was primarily to allow the fluid to serve the dual function of coolant and dielectric.
The essential problem with this approach is that it is extremely difficult to find a fluid which has both the proper coolant properties (temperature, heat of vaporization, pressure, viscosity) and good dielectric properties (high dielectric strength, molecular stability in the high electric field environment). Even if such a fluid could be found, there might still be problems due to the formation of droplets of the fluid at the conductor-dielectric interface, for if these droplets have a different dielectric constant than the fluid as a gas, this causes an additional gradient in the electric field. Also, the droplets may be subject to polarization and elongation by the electric field, thereby leading to electric breakdown between the conductor and shield.
In view of the foregoing, a need has arisen for an improved electrical transmission line system having means for adequately cooling the conductors thereof in an efficient manner without substantially increasing production and maintenance costs. Evaporation-cooling generally permits the use of less coolant mass flow and less expensive circulation equipment than that needed for single phase forced cooling. By periodically venting the evaporating coolant, undue pressure and heat build-up associated with unvented two-phase (vapor-liquid) flow can be avoided.